This invention relates to the control of a hydraulically actuated mechanism such as a drill feed mechanism in which the input power source is compressed gas. The power of the compressed gas is controlled and then converted into hydraulic power which is further controlled. Such control results in an efficient system for automatically regulating the hydraulic power available dependent upon the power demand of the load.
In previous air-powered hydraulically-actuated mine drills two hydraulic pumps, each driven by a radial piston air motor, were used. One such pump was used for low power operations and for example would be rated approximately four horsepower. The other pump would be used for higher power operations and would be for example rated at approximately ten horsepower. The prior system used air-stall closed-center hydraulic control which subjected the hydraulic pumps to extremely severe duty cycles. Frequently, operators failed to shut off the air supply to the air motors driving the pumps during no load conditions. Internal pump slippage occurred because of the continued attempt by the air motors to rotate the pump during stall. Since the pumps could not rotate because of the closed center hydraulic system, except for internal slippage, they overheated, reducing oil viscosity and increasing pump slippage still more. In addition, high operating temperature in mines coupled with improper viscosity oil caused pumps to fail frequently. Another consideration was the need for greater efficiency in the drilling cycle which represented 98% of the operating time, not counting drill rod changing time. The low power pump far exceeded the flow and power required for the drill feed part of the cycle; and the high power pump was used only during retraction when removing a rod from the drill string which represented 2% of the cycle. The open center hydraulic two pump hydraulic system often resulted in an inefficient, noisy operation suffering frequent downtime due to pump failure.